Prairie du Sac, Wis. — It may be two to three weeks before fish
biologists can determine the extent of a July 16 walleye kill below
the dam in this Wisconsin River town.
DNR fisheries biologist Gene VanDyck investigated the kill and
said the walleye loss is related to hot weather and the
“There was a real problem with dissolved oxygen levels in Lake
Wisconsin and water coming out of Lake Wisconsin,” VanDyck said.
“It appears that hydrogen sulfide came out of Lake Wisconsin, along
with the water having low dissolved oxygen. I think that (the
deadly situation) took place fairly rapidly and then subsided.”
VanDyck and other DNR personnel picked up and/or observed about
100 walleyes from 9 to 24 inches. None were floating on the
surface. All were “beached” in the shallows; they were still in the
water, but were not floating.
That has VanDyck wondering if more fish could have died, but
ended up sinking instead of floating to the surface where they
might have been found by anglers or DNR employees.
“Right now I can’t say just how serious of a loss this is,”
VanDyck said. “Obviously, if you lose 100 to 200 nice walleyes from
anywhere, it’s serious. But, if that’s all – then it’s not
catastrophic. There has been no chance to get on the river yet with
a shocker boat to see what’s out there. Right now, the biggest
priority is finishing the report and the analysis. We will make a
survey, but it could be a week off.”
If all of the collected fish had been floating, there is a
chance that those fish would then represent a fair sample of the
walleyes that were killed.
“If they were not floating and beached themselves, there’s no
telling how many (might have died). Really, there’s no way to know.
There’s 30 feet of water out there. I didn’t see a fish that wasn’t
partially beached,” VanDyck said.
If more fish died and then sank, the DNR might get an idea as to
the extent of that scenario as time goes by if those fish “gas up”
and float to the surface. In the meantime, VanDyck is trying to
line up a shocker boat and crew to run a survey of the Lower
An angler noticed floundering walleyes below the Prairie du Sac
dam on Sunday night, July 16. He told a conservation warden he was
able to pick up the walleyes by hand – they were alive, but
lethargic. He called DNR warden John Buss, of Sauk City, the next
day, and Buss called VanDyck. The fisherman said he noticed a
definite sulfur smell in the air that night.
VanDyck said hydrogen sulfide is created in water when there is
a high rate of decomposing organic matter and a low level of
“You have to have a lack of oxygen and if you have warmer water,
it accelerates the rate at which it occurs. At this time, we don’t
know why we hit that critical combination on Sunday night,” VanDyck
“The unusual thing is that the kill took place in a small window
of time and it was just walleyes. There were all kinds of fish up
here, and some with higher oxygen requirements than walleyes,” he
said. “That would often indicate a disease, but it’s not. I checked
the fish – it’s not a disease. Even if it was a disease, they
wouldn’t all die at once.
“Maybe the other fish fled from the tail race, and the walleyes
stayed – maybe the walleyes were desperate to get into
The kill occurred right below the dam at Prairie du Sac. It was
centered on the water exiting the dam.
Whatever caused the kill had already occurred and dissipated 24
hours later. The hydrogen sulfide smell that had been there was
gone when VanDyck arrived.
“(The area) was critically low on oxygen, but there are fish
back in there,” he said.
The water VanDyck collected was running at 2.5 parts per million
(ppm) of dissolved oxygen – 8 ppm would be a healthy level.
“There wasn’t anything anyone could have done to stop it, even
if we knew right away,” he said. “We might have had a better shot
at understanding what happened, but you weren’t going to stop
VanDyck is going to continue to gather information, including
“flow regimes” of water released from Lake Wisconsin, to see if
there is a way to avoid problems in the future. He also wants to
make an assessment on the severity of the kill.
“I’m just hoping it wasn’t a major portion of that population.
There was no sign of problems two miles below the dam. It may have
only affected fish in tail race,” he said.